Diffuser arrangement

ABSTRACT

A diffuser arrangement through which a fluid may flow is provided. The diffuser includes an outer diffuser comprising an inner surface, and a flow-guiding device, which is configured such that at least part of the boundary layer flow forming on the inner surface of the outer diffuser can be accelerated in the main flow direction, so that a flow separation is prevented on the inner surface of the outer diffuser. Also provided are an exhaust steam plenum of a steam turbine and an exhaust gas plenum of a gas turbine, both including a diffuser arrangement.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2008/052222, filed Feb. 25, 2008 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 07005175.0 EP filed Mar. 13, 2007, both of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention refers to a diffuser arrangement and especially to an exhaust steam plenum of a steam turbine or an exhaust gas plenum of a gas turbine with the diffuser arrangement.

BACKGROUND OF INVENTION

A diffuser is a passage which is exposable to throughflow by fluid and which in the case of separation-free throughflow decelerates the fluid by means of cross-sectional widening and, in accordance with Bernoulli's theorem, reduces the kinetic pressure of the fluid to the benefit of the static pressure.

The quality of the diffuser is described by means of the pressure recovery coefficient, which is defined by

C _(p)=(p _(out) −p _(in))/(p _(total,in) −p _(in))

and the overall or total pressure p_(total,in), the static pressure p_(in) at the diffuser inlet and the static pressure p_(out) at the diffuser outlet.

Diffusers are used for example in pipelines for pressure recovery or for constant bridging of cross-sectional widenings (transition diffuser). In the case of pipelines with circular cross section the diffusers are axially symmetrically formed. In FIG. 4, a longitudinal section of an axially symmetrical diffuser 101 is shown, and schematically shows the flow which typically occurs within it. The diffuser 101 has an inlet cross section 102 and an outlet cross section 103, the area ratio of which is greater than one. Upstream of the diffuser 101, a cylindrical inflow pipe is arranged, through which flows an inflow 108, and downstream of the diffuser 101 a cylindrical outflow pipe is arranged, through which flows an outflow 109.

On account of the adherence of the fluid on the diffuser wall, a boundary layer develops in the flow close to the wall. In the lower half of the diffuser 101 which is shown in FIG. 4, five characteristic velocity profiles 110 to 114 are shown along the main flow direction, wherein the first of the two velocity profiles 110, 111 show the flow close to the wall in the inflow pipe and the three velocity profiles 112 to 114 which follow upstream show the flow close to the wall in the diffuser 101.

Since the flow in the diffuser 101 is decelerated, the flow velocity of the main flow decreases in the flow direction, as a result of which by fulfilling the first main theorem of thermodynamics the static pressure of the flow correspondingly increases in the flow direction. According to Prandtl's boundary layer theory, the static pressure in the boundary layer is constant transversely to the flow direction.

On account of the deceleration effect of the diffuser 101 and the adherence condition on the diffuser wall, the flow velocity of the flow close to the wall decreases. After negotiating a specific flow path the gradient of the flow velocity transversely to, and on, the diffuser wall is zero. This position is a separation point 105 of the flow, which is shown in the boundary layer profile 113.

At the separation point 105, the flow moves away from the diffuser wall towards the middle of the diffuser 101, wherein downstream of the separation point 105 in the wall proximity a backflow develops which forms a separation bubble 106. The separation bubble 106 brings about a narrowing of the cross section of the diffuser 101 which is effectively exposed to throughflow so that the main flow in the region of the separation bubble 106 is accelerated. As a result, in the main flow the kinetic energy is increased and the flow is reapplied in the outlet pipe at a reapplication point 107.

The degree of opening of the diffuser 101 which is shown in FIG. 4 substantially determines the shape and the size of the separation bubble 106 and the position of the separation point 105 and of the reapplication point 107 which possibly occurs. The higher the degree of opening of the diffuser 101, the further upstream the separation point 105 lies.

On account of the narrowing of the effective cross section of the diffuser 101, the separation bubble 106 reduces the pressure recovery effect of the diffuser 101 compared with a diffuser in which the flow is fully applied.

In order to create a laminar boundary layer flow in a diffuser, a bladed wheel which is seated on a hub, the blades of which, being shrouded by a diffuser plate, are connected on the blade tip side by means of a ring, is known from laid-open specification DE 1 628 337. A ring of stator blades is arranged on the ring in such a way that this widens the jet flow which flows off the bladed wheel while maintaining the boundary flow which is guided by the diffuser plate. In addition to the stator blades, this is especially achieved by the ring having a corresponding cross-sectional shape which, moreover, benefits the course of the entrained flow filaments and blows these out at higher velocity.

Furthermore, for avoiding flow separations in a diffuser, a pipe which is arranged parallel to the diffuser wall and which extends along the flow direction, is known from JP 08 260905. On account of the diverging cross section of the diffuser and of the correspondingly diverging pipe which is parallel to it, the flow cross section of the annular passage which is formed between diffuser wall and the pipe is increased so that medium flowing in the annular passage is decelerated.

A steam turbine or a gas turbine is run at partial load, base load and overload. In the construction and design of the steam turbine or gas turbine, their individual components can be geometrically designed in an optimized manner only at a single operating point for example with regard to efficiency or aerodynamic or thermodynamic effectiveness. This has the result that at other operating points, which are not identical to the design operating point, the components cannot operate in an optimum manner.

This state also applies to an exhaust steam plenum of the steam turbine or to an exhaust gas plenum of the gas turbine. The exhaust steam plenum or the exhaust gas plenum is conventionally constructed as an axial diffuser.

As a rule, the axial diffuser is geometrically designed in an optimized manner with regard to the base load so that at partial load and overload the axial diffuser cannot be operated in an optimized manner.

At the inlet of the axial diffuser, in the case of optimum design, a lower static pressure exists than at the outlet. As a result of lowering the pressure at the diffuser inlet, which at the same time represents the exit of the blading, the last rotor blade ring is brought to a higher power output.

The mass flow of the flow which flows through the axial diffuser is lower in the partial load range than in the base load range, as a result of which the average flow velocity in the axial diffuser in the base load range is higher than in the partial load range. As a result, the flow in the axial diffuser in the partial load range is more prone to separation than the flow which occurs in the axial diffuser at base load.

Therefore, the pressure recovery in the axial diffuser at partial load is lower compared with the pressure recovery at base load. This has the result that at partial load the turbine output is lowered compared with the turbine output at base load. The influence of an improvement in the pressure recovery of a gas turbine diffuser of c_(p)=0.1 was estimated by Farohki at 0.8% of the delivered turbine output. This connection is similarly applicable to axially exhausting steam turbines.

A reduction of the degree of opening of the axial diffuser could provide a remedy in this case since the flow is decelerated less sharply as a result and is therefore prone to separation to a lesser degree. However, the overall length of the axial diffuser is consequently extended, as a result of which the total overall length of the steam turbine or gas turbine is disadvantageously increased.

SUMMARY OF INVENTION

It is the object of the invention to create a diffuser arrangement, the pressure recovery of which is high and its overall length short.

The diffuser arrangement according to the invention is exposable to throughflow by fluid and has an outer diffuser which has an inner surface, and a flow-accelerating device which is installed in such a way that at least some of the boundary layer flow which develops on the inner surface of the outer diffuser can be accelerated in the main flow direction so that a flow separation on the inner surface of the outer diffuser is prevented.

If the fluid flows through the outer diffuser, then it is decelerated in the main flow direction, as a result of which the boundary layer flow which develops on the inner surface of the outer diffuser is principally prone to separation. The separation would emanate from a point at which the kinetic energy of the flow is zero.

By means of the flow-accelerating device according to the invention at least some of the flow close to the wall is accelerated so that the kinetic energy of the flow close to the wall is increased. As a result, the effect of the kinetic energy of the flow close to the wall not being zero at any point is prevented, as a result of which a flow separation on the inner surface of the outer diffuser is prevented. Therefore, the diffuser arrangement has a high pressure recovery.

Furthermore, the outer diffuser of the diffuser arrangement can have a large degree of opening without a flow separation occurring in it. Consequently, the outer diffuser and therefore the diffuser arrangement has a shorter overall length.

The flow-accelerating device has a flow guiding device which extends inside the outer diffuser, and by its outer surface, which faces the inner surface of the outer diffuser, and a section of the inner surface of the outer diffuser, forms a nozzle passage through which the part of the boundary layer flow can flow.

Therefore, the flow-accelerating device is formed by the nozzle passage which is defined by the flow guiding device interacting with the inner wall of the outer diffuser. As a result, the effect is achieved of the flow close to the wall, i.e. just the flow portion with otherwise low kinetic energy, being accelerated directly on the inner surface of the outer diffuser. Consequently, a separation in the diffuser arrangement is effectively prevented.

Furthermore, the extent of the flow-guiding device in the main flow direction lies in the region of 5% to 40% of the extent in the main flow direction of the outer diffuser. As a result, the flow-guiding device is arranged entirely inside the outer diffuser and can be accurately placed on any section on the inner wall of the outer diffuser on which a separation of the fluid flow is to be expected. Therefore, the flow-guiding device can be purposefully arranged on a section where separation is a risk, as a result of which an effective prevention of flow separation is achieved and therefore the disturbance of the main flow by the flow-guiding device is low.

It is preferred that the flow-guiding device by its inner surface which faces away from the outer surface forms an inner diffuser through which the fluid flow can flow and in so doing can be decelerated in the main flow direction.

Therefore, in addition to the nozzle effect in the outer region the flow-guiding device also has a diffuser effect in the inner region so that the flow through the diffuser arrangement is sharply decelerated. As a result, the effect is achieved of the pressure recovery of the diffuser arrangement according to the invention being high.

It is preferred that the outer diffuser and the flow-guiding device are axially symmetrically formed and are concentrically arranged around a common symmetry axis.

Furthermore, it is preferred that the nozzle passage is formed as an annular passage.

From this, the diffuser arrangement is advantageously created as an arrangement of a plurality of diffusers and a nozzle. This arrangement is formed by a series-connecting of the three diffusers, specifically the region of the outer diffuser upstream of the flow-guiding device, the inner diffuser of the flow-guiding device, and the region of the outer diffuser downstream of the flow-guiding device, and a parallel-connecting of the nozzle passage to the inner diffuser of the flow-guiding device. As a result, a compact, simple and effectively operating division of the outer diffuser is achieved, wherein the diffuser arrangement has a compact type of construction.

The flow-guiding device is preferably formed as a straight guide plate.

As a result, the guide plate can advantageously be cost-effectively produced.

Alternatively to this, it is preferred that the flow-guiding device is aerodynamically profiled. Consequently, the flow-guiding device has a low flow resistance.

Furthermore, it is preferred that the flow-guiding device is arranged in the region of 80% to 100% of the passage height (radius) of the outer diffuser.

Consequently, the flow-guiding device is advantageously effectively placed in the flow close to the wall and, as a result, is aerodynamically effectively placed.

Furthermore, the flow-guiding device is preferably arranged in the region of the inlet cross section of the outer diffuser.

As a result, it is advantageously made possible for the inlet flow into the outer diffuser from the flow-guiding device to already have an accelerated flow in the boundary layer region, which accelerated flow over the course along the inner surface of the outer diffuser is not therefore prone to separation.

Furthermore, it is preferred that the flow-guiding device is pivotably mounted relative to the main flow.

In this way, the effect is advantageously achieved of the flow-guiding device being able to be individually adjusted by pivoting with regard to the respective flow conditions inside the outer diffuser in such a way that the flow-guiding device is aerodynamically effective.

An exhaust steam plenum of a steam turbine or an exhaust gas plenum of a gas turbine preferably features the diffuser arrangement according to the invention.

Furthermore, it is preferred that the flow-accelerating device is arranged on the inner surface of the outer diffuser in the region of its inlet.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of a diffuser arrangement according to the invention, with reference to the attached schematic drawings, are explained in the following text. In the drawing:

FIG. 1 shows a longitudinal section through a first exemplary embodiment of the diffuser arrangement,

FIG. 2 shows a longitudinal section through a second exemplary embodiment of the diffuser arrangement,

FIG. 3 shows a longitudinal section through a third exemplary embodiment of the diffuser arrangement, and

FIG. 4 shows a longitudinal section of a diffuser with schematic representation of the flow conditions.

DETAILED DESCRIPTION OF INVENTION

As is apparent from FIG. 1, a diffuser arrangement 1 has an outer diffuser 2 which is axially symmetrically formed around its symmetry axis 3. An inlet cross section 4 of the outer diffuser 2, through which an inflow 5 flows into the outer diffuser 2, lies in a plane which is perpendicular to the symmetry axis 3, and its outlet cross section 6, from which an outflow 7 discharges from the outer diffuser 2, lies in another plane which is perpendicular to the symmetry axis 3 of the outer diffuser 2. This outer diffuser has an inner surface 8 which delimits the inside space of the said outer diffuser 2.

The outer diffuser 2 is formed as a straight diffuser, i.e. the inner surface 8 of the outer diffuser 2 forms a truncated cone, wherein the cross-sectional area at the inlet cross section 4 is smaller than the cross-sectional area at the outlet cross section 6.

A flow-guiding device 9 is arranged inside the outer diffuser 2. The flow-guiding device 9 is formed as a guide plate which is oblong in longitudinal section and which, axially-symmetrically arranged around the symmetry axis 3 of the outer diffuser 2 concentrically with the outer diffuser 2, delimits a truncated cone-shaped annular passage which narrows in the flow direction.

On its outer periphery the flow-guiding device 9 has an outer surface 10 which with regard to the inner surface 8 of the outer diffuser 2 is inclined in such a way that the annulus cross section decreases in the flow direction in a plane which is perpendicular to the symmetry axis 3 and formed between the flow-guiding device 9 and the outer diffuser 2.

That is to say, the outer surface 10 of the flow-guiding device 9 interacts with a section of the inner surface 8 of the outer diffuser 2 which lies opposite it in such a way that the annular passage, which lies between the flow-guiding device 9 and the outer diffuser 2, forms a nozzle passage 11. Therefore, the section of the inner surface 8 of the outer diffuser 2 which faces the outer surface 10 of the flow-guiding device 9 is an inner surface 12 of the nozzle passage 11.

Upstream, the flow-guiding device 9 is delimited by its leading edge 13 and downstream is delimited by its trailing edge 14. An inlet cross section 15 of the nozzle passage 11 is located in the region from the leading edge 13 of the flow-guiding device 9 up to the inner surface 8 of the outer diffuser 2, and the outlet cross section 16 of the nozzle passage 11 is located in the region of the trailing edge 14 of the flow-guiding device 9 up to the inner surface 8 of the outer diffuser 2, wherein the cross-sectional area of the inlet cross section 15 is greater than the cross-sectional area of the outlet cross section 16.

Facing away from the outer surface 10 of the flow-guiding device 9, this has an inner surface 17 which forms an inner diffuser 18. The leading edge 13 of the flow-guiding device 9 is arranged in a plane which is perpendicular to the symmetry axis 3 and forms an inlet cross section 19 of the inner diffuser 18, and the trailing edge 14 of the flow-guiding device 9 is arranged in a plane which is perpendicular to the symmetry axis 3 and forms an outlet cross section 20 of the inner diffuser 18, wherein the inlet cross section 19 is smaller than the outlet cross section 20.

From FIG. 2, the aerodynamic effectiveness of the flow-guiding device 9 is evident. According to FIG. 2, the flow-guiding device 9 is formed as a profiled annular guide plate.

For representing the flow conditions in the diffuser arrangement 1, in FIG. 2 flow lines 21 are drawn in the region of the flow-guiding device 9, and a velocity profile 22 upstream of the flow-guiding device 9, a velocity profile 23 at the trailing edge 14 of the flow-guiding device 9, and also a velocity profile 24 downstream of the flow-guiding device 9 are shown.

The flow lines 21 have a converging path in the main flow direction, as a result of which the flow acceleration which is induced by means of the flow-guiding device 9 is indicated. The velocity gradient, which is normal to the wall, on the inner surface 8 of the outer diffuser 2 is flatter in the case of the velocity profile 22 upstream of the flow-guiding device 9 than in the case of the velocity profile 23 at the trailing edge 14 of the flow-guiding device 9, which is flatter than the velocity gradient, which is normal to the wall, of the velocity profile 24 downstream of the flow-guiding device 9.

Consequently, it is shown that the flow, which is guided by the flow-guiding device 9 through the nozzle passage 11, is accelerated (energized). Therefore, the flow-guiding device 9 locally increases the velocity of the flow in the proximity of the inner surface 12 of the outer diffuser 2. In the process, high-energy flow material from the core flow is deflected in the direction towards the inner surface 12 of the outer diffuser 2 and therefore is added to the boundary layer on the inner surface 12 of the outer diffuser 2. As a result of this energizing, the boundary layer on the inner surface 12 of the outer diffuser 2 can overcome greater positive pressure gradients in the main flow direction without being separated from the inner surface 12 of the outer diffuser 2 in the process.

As a result, the outer diffuser 2 reacts kindly to premature separation phenomena. Therefore, by provision of the flow-guiding device 9 in the outer diffuser 2 a higher pressure recovery of the outer diffuser 2 is achieved.

FIG. 3 shows an exhaust gas plenum of a gas turbine, which is formed as the outer diffuser 2. The outer diffuser 2 is arranged downstream of a turbine rotor 25 and guides away the outflow, which issues from the turbine rotor 25, from the inlet cross section 4 of the outer diffuser 2 to the outlet cross section 6 of the outer diffuser 2, recovering pressure.

The turbine rotor 25 has a turbine rotor hub 26 which is continued by a cylindrical outer diffuser hub 27 with the turbine rotor hub 26.

The turbine rotor 25 has a multiplicity of turbine rotor blades 28 which on their radial outer ends have a blade tip 29. The turbine rotor 25 is enclosed by a turbine casing 30. During operation of the turbine rotor 25 this rotates around its rotational axis (not shown), while the turbine casing 30 remains stationary. Therefore, a gap 31 is provided between the turbine rotor blade tip 29 and the turbine casing 30 so that the turbine rotor blade tip 29 does not rub on the turbine casing 30 during operation of the turbine rotor 25.

In order to avoid rubbing of the rotor blades on the turbine casing 30 and to thereby avoid damage, a minimum distance as a gap 31, the so-called clearance, is necessary between rotor blade 28 and casing 30. Some of the mass flow can flow through this gap without power yield to the rotor blade 28 and leads to energizing of the boundary layer. Depending upon the configuration of this gap 31, with or without sealing, mass flow can flow through to a greater or lesser extent. In order to avoid, or greatly delay, a subsequent separation of the flow in the diffuser, a further energizing of the boundary layer by means of the flow-guiding device 9 is desired.

According to FIG. 3, a remedy is provided by arranging the flow-guiding device 9 close to the inner surface 8 of the outer diffuser 2 in the region of the inlet cross section of the outer diffuser 4. The boundary layer which is disturbed by the leakage flow is accelerated in the main flow direction by the flow-guiding device 9 on the inner surface of the outer diffuser 2 so that the kinetic energy in this flow region is increased. As a result, the effect is achieved of the flow not separating in the outer diffuser 2 on the inner surface 8 of the outer diffuser 2. Therefore, the flow losses in the outer diffuser 2 are low and the pressure recovery of the outer diffuser 2 is high. 

1.-12. (canceled)
 13. A diffuser arrangement which is exposed to a throughflow by a fluid, comprising: an outer diffuser including an inner surface; and a stationary flow-guiding device, the flow-guiding device extending inside the outer diffuser, wherein a first length of the flow-guiding device lies in a range between 5% to 40% of a second length of the outer diffuser in the main flow direction, and wherein the flow-guiding device may be used as a flow-accelerating device whereby a first outer surface of the flow-guiding device, which faces a second inner surface of the outer diffuser, and a section of the second inner surface foams a nozzle passage with the result that at least some of a boundary layer flow which develops on the second inner surface may be accelerated in the main flow direction so that a flow separation on the second inner surface is prevented.
 14. The diffuser arrangement as claimed in claim 13, wherein the flow-guiding device includes a leading edge which is exposed to an inflow of the fluid and a trailing edge which lies opposite the leading edge and on which the fluid flows off, wherein an inlet cross section is located between the second inner surface and the leading edge, and an outlet cross section is located between the second inner surface and the trailing edge, and wherein a first cross-sectional area of the inlet cross section is greater than a second cross-sectional area of the outlet cross section.
 15. The diffuser arrangement as claimed in claim 13, wherein the flow-guiding device includes a first inner surface, facing away from the first outer surface, and by the first inner surface an inner diffuser is formed through which the fluid flows and may be decelerated in the main flow direction.
 16. The diffuser arrangement as claimed in claim 13, wherein the outer diffuser and the flow-guiding device are axially symmetrically formed and are concentrically arranged around a common symmetry axis.
 17. The diffuser arrangement as claimed in claim 13, wherein the nozzle passage is formed as an annular passage.
 18. The diffuser arrangement as claimed in claim 13, wherein the flow-guiding device is formed as a straight guide plate which is oblong in a longitudinal section or the flow-guiding device is aerodynamically profiled.
 19. The diffuser arrangement as claimed in claim 13, wherein the flow-guiding device is arranged in the range between 80% to 100% of a passage radius of the outer diffuser.
 20. The diffuser arrangement as claimed in claim 13, wherein the flow-guiding device is arranged in a region of the inlet cross section of the outer diffuser.
 21. The diffuser arrangement as claimed in claim 13, wherein the outer diffuser is formed as an essentially straight diffuser.
 22. The diffuser arrangement as claimed in claim 13, wherein the flow-guiding device is pivotably mounted relative to the main flow direction.
 23. An exhaust steam plenum of a steam turbine, comprising: a diffuser arrangement, comprising: an outer diffuser including an inner surface, and a stationary flow-guiding device, the flow-guiding device extending inside the outer diffuser, wherein a first length of the flow-guiding device lies in a range between 5% to 40% of a second length of the outer diffuser in the main flow direction, and wherein the flow-guiding device may be used as a flow-accelerating device whereby a first outer surface of the flow-guiding device, which faces a second inner surface of the outer diffuser, and a section of the second inner surface fauns a nozzle passage with the result that at least some of a boundary layer flow which develops on the second inner surface may be accelerated in the main flow direction so that a flow separation on the second inner surface is prevented.
 24. The exhaust steam plenum of a steam turbine as claimed in claim 23, wherein the flow-guiding device includes a leading edge which is exposed to an inflow of the fluid and a trailing edge which lies opposite the leading edge and on which the fluid flows off, wherein an inlet cross section is located between the second inner surface and the leading edge, and an outlet cross section is located between the second inner surface and the trailing edge, and wherein a first cross-sectional area of the inlet cross section is greater than a second cross-sectional area of the outlet cross section.
 25. The exhaust steam plenum of a steam turbine as claimed in claim 23, wherein the flow-guiding device includes a first inner surface, facing away from the first outer surface, and by the first inner surface an inner diffuser is formed through which the fluid flows and may be decelerated in the main flow direction.
 26. The exhaust steam plenum of a steam turbine as claimed in claim 23, wherein the outer diffuser and the flow-guiding device are axially symmetrically formed and are concentrically arranged around a common symmetry axis.
 27. The exhaust steam plenum of a steam turbine as claimed in claim 23, wherein the nozzle passage is foamed as an annular passage.
 28. An exhaust gas plenum of a gas turbine, comprising: a diffuser arrangement, comprising: an outer diffuser including an inner surface, and a stationary flow-guiding device, the flow-guiding device extending inside the outer diffuser, wherein a first length of the flow-guiding device lies in a range between 5% to 40% of a second length of the outer diffuser in the main flow direction, and wherein the flow-guiding device may be used as a flow-accelerating device whereby a first outer surface of the flow-guiding device, which faces a second inner surface of the outer diffuser, and a section of the second inner surface forms a nozzle passage with the result that at least some of a boundary layer flow which develops on the second inner surface may be accelerated in the main flow direction so that a flow separation on the second inner surface is prevented.
 29. The exhaust gas plenum of a gas turbine as claimed in claim 28, wherein the flow-guiding device includes a leading edge which is exposed to an inflow of the fluid and a trailing edge which lies opposite the leading edge and on which the fluid flows off, wherein an inlet cross section is located between the second inner surface and the leading edge, and an outlet cross section is located between the second inner surface and the trailing edge, and wherein a first cross-sectional area of the inlet cross section is greater than a second cross-sectional area of the outlet cross section.
 30. The exhaust gas plenum of a gas turbine as claimed in claim 28, wherein the flow-guiding device includes a first inner surface, facing away from the first outer surface, and by the first inner surface an inner diffuser is formed through which the fluid flows and may be decelerated in the main flow direction.
 31. The exhaust gas plenum of a gas turbine as claimed in claim 28, wherein the outer diffuser and the flow-guiding device are axially symmetrically formed and are concentrically arranged around a common symmetry axis.
 32. The exhaust gas plenum of a gas turbine as claimed in claim 28, wherein the nozzle passage is formed as an annular passage. 